Pseudo-Lucian talks about the prophecy and piety of the Magi (along with ethnographies of other tribes) as part of a discussion of life-expectancy: why do some groups of people live longer than others? Pseudo-Lucian thinks you could live a long time too if you just exercised and ate properly. Inspired by this article at the BBC: "We learn nothing about nutrition, claim medical students".
"Stories have been told of whole races who have a long life expectancy because of their way of life.* There are those called the 'Sacred Scribes' in Egypt, the 'Interpreters of Myths' in Assyria and Arabia, those called 'Brahmans' in India, men completely devoted to the study of philosophy, and those called the 'Magi', a prophetic race who dedicate themselves to the gods, found among the Persians, Parthians, Bactrians, Chorasmians, Arians, Sacae, Medes, and many other Barbarians. They are strong and long-lived because they lead their life in strict observance of magic.
"There are also whole nations who have a long life expectancy, like in the stories they tell about the Seres, who live up to three-hundred years, some attributing the cause of their old age to the climate, others to the land, and still others to their way of life: for, they say, this is a whole nation of water-drinkers. Stories are also told of the people of Athos living up to one- hundred and thirty years, and there is a report that the Chaldeans live over one-hundred years, using bread made from barley because it acts as a drug for preserving good eyesight. They also say that on account of this diet, their other senses are better than those of other people.
"But that's enough about the life expectancy of these races and nations, those which they say survive for a long time either because of the land and climate, or way of life, or both. I, however, want to propose to you that your hopes [of a long life] are easy to achieve, by telling you about men in every land and in every climate who have become long-lived by using the right kind of exercises and a healthy diet."
καὶ γένη δὲ ὅλα μακρόβια ἱστορεῖται διὰ τὴν δίαιταν, ὥσπερ Αἰγυπτίων οἱ καλούμενοι ἱερογραμματεῖς, Ἀσσυρίων δὲ καὶ Ἀράβων οἱ ἐξηγηταὶ τῶν μύθων, Ἰνδῶν δὲ οἱ καλούμενοι Βραχμᾶνες, ἄνδρες ἀκριβῶς φιλοσοφίᾳ σχολάζοντες, καὶ οἱ καλούμενοι δὲ μάγοι, γένος τοῦτο μαντικὸν καὶ θεοῖς ἀνακείμενον παρά τε Πέρσαις καὶ Πάρθοις καὶ Βάκτροις καὶ Χωρασμίοις καὶ Ἀρείοις καὶ Σάκαις καὶ Μήδοις καὶ παρὰ πολλοῖς ἄλλοις βαρβάροις, ἐρρωμένοι τέ εἰσι καὶ πολυχρόνιοι διὰ τὸ μαγεύειν διαιτώμενοι καὶ αὐτοὶ ἀκριβέστερον.
ἤδη δὲ καὶ ἔθνη ὅλα μακροβιώτατα, ὥσπερ Σῆρας μὲν ἱστοροῦσι μέχρι τριακοσίων ζῆν ἐτῶν, οἱ μὲν τῷ ἀέρι, οἱ δὲ τῇ γῇ τὴν αἰτίαν τοῦ μακροῦ γήρως προστιθέντες, οἱ δὲ καὶ τῇ διαίτῃ· ὑδροποτεῖν γάρ φασι τὸ ἔθνος τοῦτο σύμπαν. καὶ Ἀθῴτας δὲ μέχρι τριάκοντα καὶ ἑκατὸν ἐτῶν βιοῦν ἱστορεῖται, καὶ τοὺς Χαλδαίους ὑπὲρ τὰ ἑκατὸν ἔτη βιοῦν λόγος, τούτους μὲν καὶ κριθίνῳ ἄρτῳ χρωμένους, ὡς ὀξυδορκίας τοῦτο φάρμακον· οἷς γέ φασι διὰ τὴν τοιαύτην δίαιταν καὶ τὰς ἄλλας αἰσθήσεις ὑπὲρ τοὺς ἄλλους ἀνθρώπους ἐρρωμένας εἶναι.
Ἀλλὰ ταῦτα μὲν περί τε τῶν μακροβίων γενῶν καὶ τῶν ἐθνῶν, ἅτινά φασιν ὡς ἐπὶ πλεῖστον διαγίγνεσθαι χρόνον, οἱ μὲν διὰ τὴν γῆν καὶ τὸν ἀέρα, οἱ δὲ διὰ τὴν δίαιταν, οἱ δὲ καὶ δι' ἄμφω. ἐγὼ δ' ἄν σοι δικαίως τὴν ἐλπίδα ῥᾳδίαν παράσχοιμι ἱστορήσας ὅτι καὶ κατὰ πᾶσαν γῆν καὶ κατὰ πάντα ἀέρα μακρόβιοι γεγόνασιν ἄνδρες οἱ γυμνασίοις τοῖς προσήκουσιν καὶ διαίτῃ τῇ ἐπιτηδειοτάτῃ πρὸς ὑγίειαν χρώμενοι.
Pseudo-Lucian, Long Lives (Marcobii), 4-8
* διὰ τὴν δίαιταν I'm translating as "way of life" or "diet" depending on the context. Keep in mind that it includes more that just what one eats, but also hygiene more generally: one's daily routine of sleep, waking, food, drink, work and sex.